That's unusual. You would want the ability to configure dispoal on a per-frame basis, not for the whole animation.....

But whatever, just set the whole thing to DISPOSE. You only have those kinds of errors when Do Not Dispose is enabled somewhere on the GIF, since Do Not Dispose causes sequential frames to be overlayed and blended ontop of previous frames, which, when done right, saves you some file size, but when done wrong, causes artifacts.

If you downloaded the GIF from FJ, then it's likely that FJ's built-in image optimizer damaged the GIF, as I have brought up before with Admin www.funnyjunk.com/report_bugs/3000#3000, but there seems to be no quick fix.

To counteract the FJ conversion issue, you can Google Reverse Image Search the GIF, download an original, unbroken version off another website, then edit THAT one in GIMP.